Embodiments according to the present invention relate generally to the relief of bodily pain in an animal, such as a human, and more specifically to the treatment of pain by action potential activation in neural fibers.
The peripheral nervous system of an animal, such as a human, is comprised generally of efferent (motor) and afferent (sensory) neural fibers. Efferent fibers generally carry motor action potentials from the central nervous system, while afferent fibers carry sensory action potentials to the central nervous system. Since the 1950's and 1960's and the codification of the gate theory, it has been generally accepted that bodily pain results from activity in nociceptive and non-nociceptive, or somatosensory, afferent nerve fibers, and the interaction of neural signals and pathways, which are influenced by several psychological and physiologic parameters. For instance, in a healthy person, action potentials transmitted along non-nociceptive fibers do not normally generate or cause a perception of pain. However, in persons experiencing chronic pain (e.g., when a person becomes overly sensitized to pain), non-noxious stimuli, and hence the activity of non-nociceptive fibers, can cause pain. This means that in a chronic pain state, sensations that would not be perceived as pain in a healthy person (e.g. light pressure or touch) may actually be perceived as painful. That is, in an individual that experiences chronic pain, the non-noxious stimuli that are sensed (transduced) by non-nociceptive receptors can lead to a perception of pain. Generally, however, while nociceptive afferent activity “opens” a gate to the transmission of sensory action potentials related to noxious input, non-nociceptive afferent activity “closes” the gate, thereby preventing or inhibiting the transmission of such sensory signals to the brain, interrupting or reducing the perception of pain.
Prior methods of stimulation of nerves for the reduction of pain, described below, have focused on the stimulation of afferent neural fibers, and such focus is perhaps understandable due to the conventional wisdom in the art for the past five decades related to gate control theory. However, prior nerve stimulation modalities used to treat pain, especially with regards to peripheral nerves, recognized a narrow treatment window between stimulation settings that may achieve desired analgesia through sensory stimulation of non-nociceptive afferents and stimulation settings that reach the threshold for discomfort or motor stimulation of efferent fibers, the latter thought to be undesirable for a number of reasons. Supplementary to such conventional wisdom, and as described in further detail below, recruitment of efferent fibers is thought to be actually beneficial in reducing pain.
The electrical stimulation of nerves, often afferent nerves, to indirectly affect the stability or performance of a physiological system can provide functional and/or therapeutic outcomes, and has been used for activating target nerves to provide therapeutic relief of pain. While prior systems and methods can provide remarkable benefits to individuals requiring therapeutic pain relief, many issues and the need for improvements still remain.
Electrical stimulation systems have been used for the relief of pain. Despite the recognition and use of electrical stimulation for the treatment of pain, widespread use of available systems is limited. Such limited use is thought to stem from a variety of factors, such as invasiveness of required surgical procedures (e.g. lead placement in epidural space of spinal cord or surgical dissection), risk of surgical complications associated with such procedures (e.g. infection, hemorrhage, neurologic injury, and/or spinal fluid leaks), the technical skill and training required to place the electrode(s), the duration of time required to place the electrode(s) correctly, the supporting equipment (e.g. imaging equipment such as fluoroscopy) required for electrode placement, risk of device complications (e.g. migration of stimulating lead or catastrophic failure, or breakage, of such lead), and/or loss of pain relief over time.
Electrical stimulation systems may be provided as either external or implantable devices, or a combination thereof, for providing electrical stimulation to activate nerves to provide therapeutic relief of pain. These “neurostimulators” are able to provide treatment and/or therapy to individual portions of the body. The operation of these devices typically includes the use of (i) an electrode placed either on the external surface of the skin, and/or (ii) a surgically implanted electrode. In most cases, one or more surface electrodes, cuff-style electrodes, paddle-style electrodes, spinal column electrodes, percutaneous leads, and/or leadless microstimulators incorporating integral electrodes, each having one or more electrodes, may be used to deliver electrical stimulation to one or more select portions of a patient's body.
One example of an electrical stimulation system used to treat pain is a transcutaneous electrical nerve stimulation (TENS) system, which has been cleared by the U.S. Food and Drug Administration (FDA) for treatment of pain. TENS systems are external neurostimulation devices that employ electrodes placed on an external skin surface to activate target afferent nerve fibers below the skin surface. Advantageously, TENS has a low rate of serious complications, but disadvantageously, it also has a relatively low (i.e., approximately 25% or less) long-term rate of success, and some of its success is attributed to a placebo effect. Additionally, TENS has low long-term patient compliance because it may cause additional discomfort by generating cutaneous pain signals due to the electrical stimulation being applied through the skin, the electrodes may be difficult to apply, and the overall system is bulky, cumbersome, and not suited for long-term use.
In addition, several clinical and technical issues associated with surface electrical stimulation have prevented it from becoming a widely accepted treatment method. First, stimulation of cutaneous pain receptors often cannot be avoided resulting in stimulation-induced pain that limits patient tolerance and compliance. Second, electrical stimulation may be delivered at a relatively high frequency to prevent stimulation-induced pain, which leads to early onset of muscle fatigue in turn preventing patients from properly using their muscle(s). Third, it is difficult to stimulate deep nerves and/or muscles with surface electrodes without stimulating overlying, more superficial nerves and/or muscles resulting in unwanted stimulation. Finally, clinical skill and intensive patient training is required to place surface electrodes reliably on a daily basis and adjust stimulation parameters to provide optimal treatment. The required daily maintenance and adjustment of a surface electrical stimulation system is a major burden on both patient and caregiver.
Other electrical stimulation systems that have been employed to treat pain include spinal cord stimulation (SCS) systems, which are also FDA approved as implantable neurostimulation devices marketed in the United States for treatment of pain. Similar to TENS, when SCS evokes paresthesias that cover a region of pain, it confirms that the location of the electrode and the stimulus intensity should be sufficient to provide pain relief and pain relief can be excellent initially, but maintaining sufficient paresthesia coverage is often a problem due to lead migration along the spinal canal.
Spinal cord stimulation is limited by the invasive procedure and the decrease in efficacy as the lead migrates. When it can produce paresthesias in the region of pain, spinal cord stimulation is typically successful initially in reducing pain, but over time the paresthesia coverage and pain reduction is often lost as the lead migrates away from its target.
Lead migration is the most common complication for SCS systems, occurring in up to 40% or more of the cases. When the lead migrates, the active contact moves farther from the target fibers and loses the ability to generate paresthesias in the target area. SCS systems attempt to address this problem by using leads with multiple contacts so that as the lead moves, the next contact in line can be selected to be the active contact. Additionally, multiple contacts can be used to guide or steer the current toward the targeted nerve fibers and away from the non-targeted nerve fibers. Although this approach may be successful, it often requires time-intensive and complex programming, adding to the overall cost of the therapy and the burden on the patient and caregiver(s).
Peripheral nerve stimulation has been attempted and may be effective in reducing pain, but it previously required specialized surgeons to place cuff- or paddle-style leads on or around the nerves in a time-consuming and invasive surgical procedure. Such prior procedures may include the use of ultrasound-guided lead placement in an attempt to avoid placement in muscle tissue in an attempt to coapt intimately an electrode surface with a target nerve, or approximately 3 millimeters or less from the nerve.
Accordingly, the art of pain reduction by neural activation would benefit from systems and methods that improve pain reduction.